Duke's Canoe Club   Duke's, steps from Waikiki Beach in the Outrigger Waikiki Hotel, is perennially crowded and noisy, but always fun. It's a good spot for people-watching in the restaurant and on the sand. Adorned with Duke Kahanamoku-related memorabilia, this casual eatery offers a bargain breakfast buffet, lunches of the sandwich and burger sort, and, at night, it's surf, turf and cocktail time. And don't forget that old American-restaurant standby, the salad bar. For value in a daily Waikiki brunch buffet, set up daily from 7 to 10:30 a.m., it is all that: $14.95 for made-to-order omelets, eggs Benedict, pancakes and waffles, breakfast meats, and all the accompaniments.  Outrigger Waikiki Hotel, 2335 Kalakaua Ave. 922-2268. 7 a.m.-midnightdaily. Reservations recommended. Valet and self-parking. AX, DC, DS, JCB, MC, VS. No checks. Breakfast and lunch: $. Buffet: $$. Dinner: $$$.  Web site: www .dukeswaikiki.com